Recently, due to shortage of energy sources and with environmental awareness on the rise, various energy saving functions attract more and more attention in applications of power supplies, of which the standby power consumption is one of the key factors. As shown in FIG. 1, a display 10 includes a power supply system 12 to provide the voltage needed by the internal components of the display 10, and an image scalar 14 to control the size of an image displayed. The power supply system 12 uses a flyback voltage converter 16 to convert an input voltage Vin to an output voltage Vo, and the voltage Vs supplied to the image scalar 14 is generated from the voltage Vo by a low-dropout (LDO) regulator 18. In the flyback voltage converter 16, a transformer Tx has a primary coil Lp connected between the power input end Vin and a power switch Qsw, and a secondary coil Ls connected between a diode Do and a ground end GND, a power controller 20 provides a pulse width modulation (PWM) signal Vgs from a pin GATE to switch the power switch Qsw and thereby control the power delivery of the transformer Tx, the transformer Tx further includes an auxiliary coil Laux connected between a ground end GND and a diode Daux to supply a current Iaux for charging a capacitor Cvdd and thereby providing electricity to a power pin VDD of the power controller 20, a photocoupler 22 and a shunt regulator 24 are used to generate a feedback signal FB to feed back the output information of the flyback voltage converter 16 to a pin COMP of the power controller 20, a current sense resistor Rcs is connected in series with the power switch Qsw to detect the current in the primary coil Lp to generate a current sense signal VCS injected to a current sense pin CS of the power controller 20, and according to the feedback signal FB and the current sense signal VCS, the power controller 20 modulates the duty of the power switch Qsw to regulate the output voltage Vo at a certain value and control the output power of the flyback voltage converter 16.
FIG. 2 is a waveform diagram for demonstrating a conventional control method of the power controller 20. As shown by the waveform 26, the output current Iload of the flyback voltage converter 16 is higher in a normal mode than in a standby mode. In the normal mode, to support a heavy loading, the power switch Qsw is switched by a high-frequency PWM signal Vgs. In the standby mode, however, the power controller 20 operates in a pulse skipping mode, in which the power switch Qsw is switched less frequently because the number of pulses is reduced, and the lighter the loading is, the more pulses are skipped. Nevertheless, the use of electricity by the image scalar 14 in the standby mode was never effectively adjusted. As shown by the waveform 28, the supply voltage Vs of the image scalar 14 remains constant in both the normal and standby modes. Therefore, even after the display 10 enters the standby mode, the image scalar 14 and the power controller 20 still operate as in the normal mode and accordingly, cause considerable power consumption. If the image scalar 14 is turned off for power saving, the display 10 will also enter an OFF mode and thus leave the standby mode.
Hence, it is desired an apparatus and method for reducing the standby power consumption of a display while maintaining normal operation of an image scalar of the display during the standby mode.